The field of the invention relates generally to railroad operation, and more specifically to determining the occupation of a section of railroad track.
Rail vehicle operators rely on information, such as whether upcoming sections of track are occupied, in order to safely and efficiently operate a rail vehicle. Currently, Direct Current (DC) track circuits and Alternating Current (AC) track circuits are used to detect the presence of rail vehicles within a defined section of track known as a track block. DC track circuits and AC track circuits use a transmitter positioned at a first boundary on a rail and a receiver positioned at a second boundary on the rail. The rail section between the first and second boundaries defines the outer limits of the track block. AC track circuits are further described as either Power Frequency (PF) or Audio Frequency (AF) track circuits based on the frequency of operation. AF track circuits operate at higher frequencies than PF track circuits. For the AF track circuit, a modulated carrier signal is transmitted into the rail at the first boundary and is received at the second boundary. If the modulated carrier signal reaches the second boundary with a signal strength that is above a predetermined level, the track block is determined to be unoccupied. In contrast, the track block is determined to be occupied when the strength of the signal received at the second boundary is below a predetermined level. For example, if a rail vehicle approaches the track block, the vehicle electrically shunts the rail, which reduces the strength of the signal received at the second boundary. A rail vehicle may be referred to as a rolling shunt because of a vehicle's effect on the track circuit.
Unlike DC track circuits, AC track circuits can be used in electrified territories. And unlike the DC and PF track circuits, the AF track circuits do not require the use of insulted rail joints at the track circuit boundaries. However, certain conditions, for example, varying electrical conductance through the ballast between the rails, and/or varying wheel/rail contact resistance, may create inconsistent signal levels at the receiver. Inconsistent signal levels at the receiver may result in an imprecise determination of the track circuit boundary location based on the energy level received from the transmitter. A fixed signal strength threshold is currently used to compensate for these limitations. The fixed signal strength threshold ensures the track circuit indicates the track block is occupied whenever a shunt is placed at either the first boundary, the second boundary, or any location between the two. For example, the fixed threshold may be set to be fifty percent (50%) of the maximum signal level. The maximum signal level occurs when the defined track block and both adjacent track blocks are not occupied. The fixed threshold approach may result in a track circuit signaling that a track block is occupied when no train is present within the track block boundaries. The perceived track circuit boundary definition may be as much as fifty feet or more beyond the physical boundaries of the track block. In other words, as a rail vehicle approaches the track block in question, the track circuit may falsely indicate it is occupied before the shunt actually enters the track block. Such a phenomenon is commonly referred to as pre-shunt phenomenon. The false indication of an occupied track block may also occur as a train is departing the track block in question. Such a phenomenon is referred to as post-shunt phenomenon.
Through technology, rails today provide information to operators through means that may be positioned along side of the rail structure, visible to the train operator (referred to as fixed wayside signals), and some that are delivered to the cab of a train for use by an operator (referred to as in-cab signals). Wayside and in-cab signals provide a train operator with information such as continue/stop instructions and suggested operating speeds. Information provided to the operator via such means are at least potentially based on whether an upcoming track block is occupied or unoccupied. If a rail vehicle approaching a track block creates a pre-shunt condition, the operator of the rail vehicle may be instructed to slow or stop the rail vehicle due to the false determination of track block occupancy. Pre-shunt and post-shunt conditions may reduce the efficiency of railroad operation.